I got tired of getting surprised by monsters in my own, explored mines due to the problem in the left 4koma, so I started building gravel walls to seal off unexplored areas and light them up with torches one branch at a time.
When I return to explore the sealed area, I take one segment of the wall down low enough to peek over it before destroying the whole wall.
Every time I do this, without fail, I get it down to one block high to find a mob staring me dead in the face like "sup". I think they know my scent.
I use cobble or dirt instead, and only have a one block hole a block above the ground to peek through. If I'm feeling fancy I might use a door if I have one on me. Sealing branches for later exploration should be one of the key skills of Minecraft, like never mining above your head and below your feet.
